There is known at present a whole range of erosion resistant coatings which operate at temperatures of up to 1260.degree. C. There is known a two-layer coating comprising a barrier layer and a glaze layer (see U.S. Pat. No. 3,953,646). The barrier layer is formed by a deposit of fused silica comprising approximately 80 to 90% by weight of solid material. The coating is applied to the substrate by spraying. The barrier layer is fired at a temperature of approximately 930 to approximately 1370.degree. C. The glaze layer, consisting of high silica glass, of borosilicate glass and of an emissivity agent, is applied to the barrier layer. The emissivity layer is selected from the group formed by silicon carbide, chromium, cobalt and nickel oxides, nickel-chromium spinels, silicon nitride and calcined, mixed oxides of iron, chromium and/or nickel. High silica glass (Corning Glass No. 7913) contains not less than 94 weight % of SiO.sub.2. The weight composition of the borosilicate glass (Corning Glass No. 7740) is as follows: 70 to 87% of SiO.sub.2 , 10 to 20% of B.sub.2 O.sub.3, 2 to 5% of Na.sub.2 O and 1 to 5% of Al.sub.2 O.sub.3.
The high silica glass component and the borosilicate glass component are used in a weight ratio of approximately 3:1 to approximately 19:1, and the glass components (high silica glass and borosilicate glass) and the emissivity agent are used in a weight ratio ranging from 50:1 to approximately 4:1. An aqueous slurry containing from approximately 10 to approximately 90 weight % of glaze coating is fired at a temperature ranging from 930.degree. to approximately 1370.degree. C.
There is known in the art (see U.S. Pat. No. 3,955,034) a three-component coating for silica insulation comprising a silica barrier layer, an emissivity layer comprising a high silica glass component and an emissivity agent selected from the group formed by silicon carbide, nickel oxide, chromium oxide, cobalt oxide, a nickel-chromium spinel, silicon nitride and calcined, mixed oxides of iron, chromium and cobalt, with a weight ratio of the high silica glass to the emissivity agent ranging from approximately 50:1 to approximately 4:1, and an overglaze layer of high silica glass and borosilicate glass in a weight ratio of high silica glass to borosilicate glass ranging from approximately 3:1 to approximately 19:1. The coating is fired at a temperature ranging from 930.degree. to approximately 1370.degree. C.
These coatings provide neither sufficient thermal shock resistance nor sufficient heat emissive stability, and they are subject to shrinkage.
To overcome the problems mentioned above, there has been proposed a one-layer coating (see U.S. Pat. 4,093,771) which is prepared by reacting a compound, selected from the group of substances formed by silicon tetraboride, silicon hexaboride, other boron silicides, boron and mixtures of these substances, with a reactive glass frit composed of high silica porous borosilicate glass and boron oxide. A thin layer of borosilicate glass is formed on finely divided particles of high silica glass, which improves the sintering of the coating without a substantial increase in the thermal expansion coefficient.
The reactive glass frit is advantageously prepared by blending approximately 2 to 10 parts by weight of boron oxide with 100 parts by weight of high silica porous borosilicate glass, such as Vycon.RTM. 7930 glass. Vycon.RTM. 7930 high silica borosilicate glass has a porosity of approximately 28%. The boron oxide is dissolved in 200 to 400 parts by weight of deionized water. The mixture is stirred at approximately 95.degree. C., and then dried for a period of up to 24 hours, at a temperature of 75.degree. to 95.degree. C. The resulting glass frit is dispersed, screened and fired at 1150.degree. C. for 1 hour. The resulting sintered composite is ground to a powder and screened.
A typical composition would be 97.5 weight % of reactive glass frit containing 5.5 weight % of boron oxide, combined with 2.5 weight % of silicon tetraboride composed of 63.+-.3 weight % of silicon, 36.+-.3 weight % of boron and less than 0.2 weight % of magnesium. The coating slurry is prepared by blending finely divided particles of reactive glass frit and silicon tetraboride, with a carrier such as ethanol and a pre-binder such as methylcellulose, in a proportion by weight of solid components of 35 to 50%. The mixture of coating components is milled in an alumina ball mill with alumina balls for 3 to 12 hours. The coating is applied by spraying. The coated samples are dried for 2 to 5 hours at temperatures in the range of 20.degree. to approximately 70.degree. C. After drying, the coated samples are glazed in an oven for 1.5 hours at 1215.degree. C. The coating has an emissivity of approximately 0.90 to 0.93 from ambient temperature to over 1260.degree. C. The thermal expansion coefficient is 1.1..multidot.10.sup.-6 K.sup.-1.
There is also known an advanced low density coating for the protection of aluminosilicate porous materials that has an operating temperature of up to 1300.degree. C. The composition of the coating comprises 77.5 weight % of reactive glass frit, 2.5 weight % of silicon tetraboride and 20 weight % of molybdenum disilicide. The coating is formed on the substrate at 1230.degree. C. for 1.5 hours (see: Advanced Porous Coating for low density Ceramic Insulation Materials, J. Amer. Ceram. Soc., vol. 72, No. 6, pages 1003-1010, 1989).
The use of high silica borosilicate glass having an active surface in the coating composition can lead to a reduction in thermochemical stability and phase stability.
Phase transformations in the coating are linked with the formation of .alpha.-cristobalite which causes cracking in the coating.